Audio apparatus

ABSTRACT

Audio apparatus ( 30 ) comprising a piezoelectric transducer ( 44 ) and coupling means ( 54 ) for coupling the transducer to a user&#39;s pinna ( 32 ) whereby the transducer excites vibration in the pinna ( 32 ) to cause it to transmit an acoustic signal from the transducer ( 44 ) to a user&#39;s inner ear, characterised in that the transducer is embedded in a casing ( 42 ) of relatively soft material and the casing ( 42 ) is mounted to a housing ( 34 ) of relatively hard material such that a cavity ( 48 ) is defined between the casing ( 42 ) and housing ( 34 ). A method of designing audio apparatus comprising mechanically coupling a piezoelectric transducer to a user&#39;s pinna and driving the transducer so that the transducer excites vibration in the pinna to cause it to transmit an acoustic signal from the transducer to a user&#39;s inner ear, characterised by embedding the transducer in a casing of relatively soft material and by mounting the casing to protective housing of relatively hard material such that a cavity is defined between the casing and housing.

TECHNICAL FIELD

The invention relates to audio apparatus and more particularly to audioapparatus for personal use.

BACKGROUND ART

It is known to provide earphones which may be inserted into a user's earcavity or headphones comprising a small loudspeaker mounted on aheadband and arranged to be placed against or over the user's ear. Suchsound sources transmit sound to a user's inner ear via the ear drumusing air pressure waves passing along the ear canal.

A typical conventional earphone uses a moving coil type transducermounted in a plastic housing. The moving coil is connected to a lightdiaphragm which is designed to fit into the entrance of the ear canal.The moving coil and diaphragm are light and are coupled intimately tothe eardrum at the other end of the ear canal. The acoustic impedance ofthe eardrum and ear canal seen by the moving coil transducer isrelatively small. This small impedance in conjunction with the intimatecoupling means that the motion requirements of the moving coiltransducer are relatively low.

A moving coil transducer requires a magnetic circuit, which typicallycontain metal parts, e.g. steel or iron pole pieces, to generatemagnetic field lines for the coil to move. These parts provide arelatively large inertial mass which combined with the low motionrequirement means that relatively little vibration enters the housing.

There are disadvantages associated with both headphones and earphones.For example, they may obstruct normal auditory process such asconversation or may prevent a user from hearing useful or importantexternal audio information, e.g. a warning. Furthermore, they aregenerally uncomfortable and if the volume of the sound being transmittedis too high they may cause auditory overload and damage.

An alternative method of supplying sound to a user's inner ear is to usebone conduction as for example in some types of hearing aids. In thiscase, a transducer is fixed to a user's mastoid bone to be mechanicallycoupled to the user's skull. Sound is then transmitted from thetransducer through the skull and directly to the cochlea or inner ear.The eardrum is not involved in this sound transmission route. Locatingthe transducer behind the ear provides good mechanical coupling.

One disadvantage is that the mechanical impedance of the skull at thelocation of the transducer is a complex function of frequency. Thus, thedesign of the transducer and the necessary electrical equalisation maybe expensive and difficult.

Alternative solutions are proposed in JP56-089200 (Matsushita ElectricInd Co Ltd), WO 01/87007 (Temco Japan Co, Ltd) and WO 02/30151 to thepresent applicant. In each publication, a transducer is coupled directto a user's pinna, in particular behind a user's earlobe, to excitevibration therein whereby an acoustic signal is transmitted to theuser's inner ear.

As set out in WO 02/30151, the transducer may be piezoelectric. Like themoving coil type transducer in a conventional earphone, thepiezoelectric transducer requires protection from mechanical damage.Furthermore, the piezoelectric transducer must be mechanically coupledto the pinna and this coupling must be protected. Accordingly, thetransducer may be mounted in a protective housing.

The piezoelectric transducer is not in intimate coupling with theeardrum and drives through the relatively high impedance of the pinna.Furthermore, sound is transmitted to the eardrum through a mechanicalcoupling rather than an audio coupling. Accordingly, a relatively highlevel of vibration energy is required to maintain the same level at theeardrum as a conventional earphone.

Unlike in a moving coil type transducer, a piezoelectric transducer doesnot have a high inertial mass to which the vibrations may be referenced.Accordingly, the housing may vibrate to produce unwanted external soundradiation. Such leakage of sound radiation may annoy nearby listenersand may reduce the privacy for the wearer and is detrimental to theperformance of the audio apparatus. Accordingly, an object of theinvention is to provide an improved design of housing.

DISCLOSURE OF INVENTION

According to a first aspect of the invention, there is provided audioapparatus comprising a piezoelectric transducer and coupling means forcoupling the transducer to a user's pinna whereby the transducer excitesvibration in the pinna to cause it to transmit an acoustic signal fromthe transducer to a user's inner ear, characterised in that thetransducer is embedded in a casing of relatively soft material and thecasing is mounted to a housing of relatively hard material such that acavity is defined between the casing and housing.

The pinna is the whole of a user's outer ear. The transducer may becoupled to a rear face of a user's pinna adjacent to a user's concha.

The casing and housing together form a two-part structure which protectsthe transducer. The use of a two-part structure provides greaterflexibility of design to create apparatus which produces minimalunwanted radiation, and has a transducer which is sufficiently protectedwith good sensitivity. In contrast, mounting a piezoelectric transducerin a one-part housing is less flexible. If a relatively hard material isused this may adversely affect the sensitivity and bandwidth of theapparatus and may lead to unwanted radiation. However, if a relativelysoft material is used, the apparatus may not be sufficiently robust.

The casing may be moulded. The relatively soft material may have a Shorehardness in the range of 10 to 100, possibly 20 to 80 and may forexample be rubber, silicone or polyurethane. The material may also benon-conducting, non-allergenic and/or waterproof. The materialpreferably has minimal effect on the performance of the transducer, i.e.does not constrain movement of the transducer and may provide someprotection, e.g. from small shocks and the environment, particularlymoisture.

The housing is preferably rigid material so as to provide extraprotection for the transducer, particularly during handling. Therelatively hard material may have a Young's modulus of 1 GPa or higherand may for example be a metal (e.g. aluminium or steel which haveYoung's moduli of 70 GPa and 207 GPa respectively), hard plastics (e.g.perspex, Acrylonitrile Butadiene Styrene (ABS) or a glass reinforcedplastic having a Young's modulus of 20 GPa) or soft plastics having aYoung's modulus of 1 GPa.

Both the casing and the housing may be moulded, e.g. in a two stepmoulding operation. Alternatively, the housing may be cast or stamped.The casing may be a snap-fit in the housing for ease of manufacture.

The coupling between the casing and the housing is preferably minimal toreduce transmission of vibration from the transducer to the housing. Thehousing may be coupled to the casing at locations on the casing havingreduced vibration. The locations may contact regions of the transducerat which vibration is suppressed, e.g. by mounting masses. The locationsmay be at the opposed ends of the casing.

The cavity may ensure minimal coupling between the casing and thehousing. The cavity may also be designed to reduce rear radiation fromthe transducer which may reduce unwanted radiation from the apparatus.The cavity may have a mechanical impedance (Z_(cavity)) which is lowerthan the output impedance of the transducer and more preferably, lowerthan the impedance of the pinna (Z_(pinna)). Thus the mechanicalimpedance of the cavity is preferably designed such that it does notlimit available force. Therefore the motion of the transducer andavailable force is not significantly effected by the cavity. Thereforethe cavity does not have a detrimental effect on the sensitivity of thedevice. Where the cavity impedance is less than the pinna impedance, allthe available force may be transmitted to the pinna and the cavity has aminimal effect on the operation of the device. The effect of the cavityis then limited to the desired function of mechanical protection andreduction of unwanted external acoustic radiation.

The mechanical properties, in particular mechanical impedance, of thetransducer may be selected to match those of a typical pinna. Bymatching the mechanical properties, in particular the mechanicalimpedance, improved efficiency and bandwidth may be achieved.Alternatively, the mechanical properties may be selected for suitabilityto the application. For example, if the matched transducer is too thinto be durable, the mechanical impedance of the transducer may beincreased to provide greater durability. Such a transducer may havereduced efficiency but may still be useable.

The mechanical properties of the transducer may be matched to optimisethe contact force between the transducer and the pinna, for example byconsidering one or more parameters selected from smoothness, bandwidthand/or level of the frequency response determined by each subjectiveuser as well as the physical comfort of the user both statically and inthe presence of an audio signal. The mechanical properties of thetransducer may be selected to optimise the frequency range of thetransducer.

The mechanical properties may include the location of the mounting,added masses, the number of piezoelectric layers. The transducer mayhave an off centre mounting whereby a torsional force is used to providegood contact to the pinna. Masses may be added, for example at the endsof the piezoelectric element, to improve the low frequency bandwidth.The transducer may have multiple layers of piezoelectric materialwhereby the voltage sensitivity may be increased and the voltagerequirement of an amplifier may be reduced. The or each layer ofpiezoelectric material may be compressed.

The coupling means preferably provide a contact pressure between thepinna and the apparatus so that the apparatus is coupled to the fullmechanical impedance of the pinna. If the contact pressure is too light,the impedance presented to the apparatus is too small and the energytransfer may be significantly reduced. The coupling means may be in theform of a hook, an upper end of which curves over an upper surface ofthe pinna. The lower end may curve under the lower surface of the pinnaor may hang straight down behind the pinna. A hook having both endscurving over the pinna may provide a more secure fitting and shouldmaintain sufficient contact pressure for efficient energy transfer.

The housing is mounted to the hook so that the transducer casingcontacts a lower part of the pinna, for example the ear lobe. The hookmay be made of metal, plastics or rubberised material.

The audio apparatus may comprise a built-in facility to locate theoptimum location of the transducer on the pinna for each individual useras taught in WO 02/30151. The audio apparatus may comprise an equaliserfor applying an equalisation to improve the acoustic performance of theaudio apparatus.

The audio apparatus may be unhanded, i.e. for use on both ears. Themanufacture may thus be simpler and cheaper since the tooling costs arereduced. Furthermore, the apparatus may be more user-friendly since auser cannot place the apparatus on the wrong ear and replacements may beeasier to obtain. A user may use two audio apparatuses, one mounted oneach ear. The signal input may be different to each audio apparatus,e.g. to create a correlated stereo image or may be the same for bothaudio apparatuses.

The audio apparatus may comprise a miniature built in microphone e.g.for a hands free telephony and/or may comprise a built in microreceiver, for example, for a wireless link to a local source e.g. a CDplayer or a telephone, or to a remote source for broadcasttransmissions.

According to a second aspect of the invention, there is provided amethod of designing audio apparatus comprising mechanically coupling apiezoelectric transducer to a user's pinna and driving the transducer sothat the transducer excites vibration in the pinna to cause it totransmit an acoustic signal from the transducer to a user's inner ear,characterised by embedding the transducer in a casing of relatively softmaterial and by mounting the casing to a protective housing ofrelatively hard material such that a cavity is defined between thecasing and housing.

The method may comprise selecting parameters of one or more of thecavity, casing and housing to reduce unwanted radiation, provideprotection for the transducer and/or to ensure good sensitivity andbandwidth. In particular, the coupling between the casing and housingand/or the cavity may be selected to reduce unwanted radiation. Thematerial of the casing may be selected to ensure good sensitivity andbandwidth and/or provide some protection for the transducer. Thematerial of the housing may be selected to provide additionalprotection. The mechanical impedance of the cavity may be lower than theoutput impedance of the transducer and more preferably, lower than theimpedance of the pinna.

The method may comprise measuring the acoustic performance of the audioapparatus for each user and adjusting the location of the transducer onthe pinna for each individual user to optimise acoustic performance, forexample to provide optimal tonal balance. The optimal position may bemeasured by determining the angle between a horizontal axis extendingthrough the entrance to the ear canal and a radial line which extendsthrough the entrance and which corresponds to the central axis of thetransducer. The angle may be in the range of 9 to 41 degrees ofdeclination.

The method may comprise applying an equalisation to improve the acousticperformance of the audio apparatus. The method may comprise applyingcompression to the signal applied the transducer, particularly if thetransducer is a piezoelectric transducer. The method may compriseoptimising the contact force between the transducer and the pinna. Thecontact force may be optimised by considering parameters such assmoothness, bandwidth and/or level of the frequency response determinedby each subjective user as well as the physical comfort of the user bothstatically and in the presence of an audio signal.

The audio apparatuses and methods described above may be used in manyapplications, for example hands free mobile phones, virtualconferencing, entertainment systems such as in-flight and computergames, communication systems for emergency and security services,underwater operations, active noise cancelling earphones, tinnitusmaskers, call centre and secretarial applications, home theatre andcinema, enhanced and shared reality including data and informationinterfaces, training applications, museums, stately homes (guided tours)and theme parks and in-car entertainment. Furthermore, the audioapparatus may be used in all applications where natural and unimpededhearing must be retained, e.g. enhanced safety for pedestrians andcyclists who are also listening to programme material via personalheadphones.

A partially deaf person may have good or adequate hearing over part ofthe frequency range and poor hearing over the rest of the frequencyrange. The audio apparatus may be used to augment the part of thefrequency range for which a partially deaf person has poor hearingwithout impeding the deaf person's hearing over the rest of thefrequency range. For example, the audio apparatus may be used to augmentthe upper frequency range for a partially deaf person who has good oradequate hearing in the lower part of the frequency spectrum or viceversa. The low frequency range may be below 500 Hz and the highfrequency range above 1 kHz.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and purely by way ofexample, specific embodiments of the invention will now be described,with reference to the accompanying drawings in which

FIG. 1 is a perspective view of an embodiment of the present inventionmounted on a pinna;

FIG. 2 is a cutaway side view of the audio apparatus of FIG. 1 withparts removed for clarity;

FIG. 3 is a cross-sectional view of the apparatus of FIG. 1, taken atright angles to that of FIG. 2;

FIGS. 4 a to 4 c are side views of alternative piezoelectric transducerswhich may be used in the present invention;

FIG. 5 is a graph of power against frequency for the transducer of FIG.4 b when attached to the pinna;

FIG. 6 is a schematic diagram of the mechanical impedances of thecomponent of an audio apparatus according to an aspect of the invention;

FIG. 7 a is a graph of the mechanical impedances of the components withfrequency;

FIG. 7 b is a simplified version of FIG. 7 a, and

FIG. 8 shows a side view of a user's ear on which an audio apparatus maybe mounted in a preferred position.

DETAILED DESCRIPTION

FIG. 1 shows an audio apparatus 30 according to the present inventionmounted on a pinna 32. The apparatus comprises a protective outerhousing 34 to which coupling means 54 having upper and lower hooks 36,38are attached. The hooks 36,38 loop over the upper and lower parts of thepinna 32 respectively to ensure a good contact between the apparatus andthe pinna. Leads 40 extend from the housing 34 to be connected to anexternal sound source.

As shown in FIGS. 2 and 3, the outer housing 34 is a hollow body whichhouses a casing 42 in which a piezoelectric transducer 44 is embedded. Acavity 48 is defined between the inner face of the outer housing 34 andthe outer face of the casing 42. The casing 42 is of generallyrectangular cross-section with a concave section 46 and is shaped so asto provide a snug fit on the user's pinna. The casing 42 is formed froma material which is much softer that the material used for the housing34.

The outer housing 34 is connected to opposed ends of the casing 42 byconnectors 50 which minimise transmission of vibration from the casing42 to the housing 34. The housing 34 is formed with loops 52 whichsecure the coupling means 54 thereto.

The casing 42 is formed with a projection 57 along the short axis whichprovides lugs 56 on either side of the casing 42. The lugs 56 engage incorresponding grooves 58 on the inner face of the outer housing 34. Innormal operation the lugs 56 are not in contact with the housing 34 butprevent the casing from being detached from the housing, e.g. if thecasing is pulled vertically. The coupling means 54 is secured to theouter face of the outer housing 34.

FIGS. 4 a to 4 c show alternative piezoelectric transducers which may beused in the present invention. In FIG. 4 a, the transducer 10 is curvedand comprises two curved piezoelectric layers 12 sandwiching a curvedshim layer 14. In FIGS. 4 b and 4 c, the transducers are not curved andare rectangular of length 28 mm and width 6 mm.

In FIG. 4 b, the transducer 80 comprises two layers, 82 of piezoelectricmaterial each of thickness 100 micron. Each piezoelectric layer 82 isseparated by a shim layer 84 of brass which is 80 micron thick. Masses86 are mounted to each end of the transducer, e.g. to suppress vibrationin the transducer at these regions. The transducer has an outputimpedance of 3.3 Ns/m. In FIG. 4 c, the transducer comprises threelayers 16 of piezoelectric material (e.g. PZT) alternating with fourelectrode layers 18 (typically silver palladium). The polarity of eachpiezoelectric layer 16 is indicated with an arrow. The layers arearranged alternately in a stack with the top and bottom layers beingelectrode layers 18. The transducer is mounted on an alloy shim 17 andis secured by an adhesive layer 19.

FIG. 5 shows a measurement of the power dissipated in the transducer ofFIG. 4 b when it is attached to the pinna (dotted line) and when it isnot attached to the pinna (solid line). When the transducer is mountedto the pinna the power extracted from the transducer is increased sincethe load of the pinna significantly increases the real part of theelectrical impedance of the transducer. Generally, the electricalimpedance of a piezoelectric element is predominately capacitive.

The cavity may be designed as set out below with reference to FIGS. 6 to7B. FIG. 6 shows a schematic diagram of the impedances of the system,namely the impedances of the pinna 32, the transducer 70, the cavity 72and the outer housing 74. The cavity has a stiffness or mechanicalimpedance determined by its area and depth. A vibration of the outerhousing 74 or casing around the transducer leads to compression of thisstiffness and thus the housing and casing may be considered to becoupled to the cavity. The mechanical impedance of the cavity may beestimated by calculating the compliance of an air-load which itself maybe estimated (assuming small displacements) from:$C_{cavity} = \frac{depth}{{Area} \cdot P_{0}}$where P₀ is atmospheric pressure (101 kPa).

The mechanical impedance of the cavity may then be expressed over afrequency range using:$Z_{cavity} = \frac{1}{2 \cdot \pi \cdot f \cdot C}$

The parameters (e.g. size and composition) of the piezoelectrictransducer are selected for efficient energy transfer to the mechanicalimpedance of the pinna over a given bandwidth. One acceptable design oftransducer which operates from 500 Hz to 10 kHz comprises fivepiezoelectric layers and is 28 mm×6 mm. Such a transducer has amechanical output impedance of 4.47 kg/s. A cavity with the same area asthe transducer and a depth of 2.5 mm has an air-load compliance of1.47×10-4 m/N.

FIG. 7 a shows the impedance of the cavity (Zcavity), the pinna (Zpinna)and the transducer (Zpiezo) against frequency. The impedance of thepinna is roughly constant with frequency below 1 kHz at a value ofZpinna=2.7 kg/s. Accordingly, the impedance of each component may besimplified as shown in FIG. 7 b. At a frequency f₁ (approx. 420 Hz) themechanical impedance of the cavity is equal to that of the transducer.Below this frequency the transducer output will be constrained by theaction of the cavity and thus f₁ should be set as the minimum operatingfrequency for the apparatus. The frequency of f₁ may be lowered byincreasing the size (particularly depth) of the cavity to avoid thecrossover point occurring in the working band of the apparatus. Makingthe cavity deep enough minimises the coupling between the casing and/orhousing and the cavity in the frequency band of interest.

At the lowest operating frequency, namely 500 Hz, Zcavity=2.17 kg/s andthus Zcavity<Zpiezo and Zcavity<Zpinna. This condition is also satsifiedthroughout the operating frequency, i.e. up to 10 kHz, since Zpiezo isconstant, Zpinna is constant to 1 kHz and then rises whereas Zcavitydecreases with frequency.

FIG. 8 shows how the location of the transducer on the pinna may beadjusted for each individual user to provide optimal tonal balance or tooptimise other features of the acoustic response. By optimising thelocation of the transducer, the pinna and the transducer may in effectform a combined driver which is unique to an individual user. Theoptimal position is measured by determining the angle θ between acentral radial line 62 and a horizontal axis 66 both extending throughthe entrance 60 to the ear canal. The central radial line 62 correspondsto the central axis of the transducer and gives the optimal position forthe transducer for a first user.

Upper and lower radial lines 64, 65 both at an angle α to the centralradial line 62 show the extent of possible deviation from the centralradial line 62 which may lead to the optimum position for a second user.Tests have been conducted which give a value for θ of 25° and for α of16°. The audio apparatus may comprise a built-in facility to locate theoptimum position. The adjustment to the angle may be made by combinedmovement of the transducer and upper end of the hook. As an alternativeto using the horizontal axis, the angle may be measured relative to avertical axis 68 extending through the entrance 60 to the ear canal.

By mounting the transducer behind the ear, the audio apparatus isunobtrusive, discreet, and does not obstruct or distort the shape of thepinna. The transducer is distanced from and thus does not impede theentrance to the ear canal and thus normal hearing is not affected.Furthermore, there is reduced occlusion of the external ear and hencereduced or no localisation errors when compared to conventionalheadphones which occlude the ear to varying degrees.

The audio apparatus may be manufactured from low cost, lightweightmaterials and may thus be disposable. The disposability may be anadvantage where hygiene is paramount, e.g. conference use.Alternatively, since the audio is not inserted into the ear, it may bemore comfortable and thus more suitable for long term wear.

1. Audio apparatus comprising a piezoelectric transducer and a couplingadapted to couple the transducer to a user's pinna whereby thetransducer excites vibration in the pinna to cause it to transmit anacoustic signal from the transducer to a user's inner ear, characterisedin that the transducer is embedded in a casing of relatively softmaterial and the casing is mounted to a housing of relatively hardmaterial such that a cavity is defined between the casing and housing.2. Audio apparatus according to claim 1, wherein the transducer isadapted be coupled to a rear face of a user's pinna adjacent to theuser's concha.
 3. Audio apparatus according to claim 1, wherein thecoupling between the casing and the housing is minimal to reducetransmission of vibration from the transducer to the housing, andwherein the housing is coupled to the casing at locations on the casinghaving reduced vibration.
 4. Audio apparatus according to claim 3,wherein the locations contact regions of the transducer at whichvibration is suppressed.
 5. Audio apparatus according to claim 3,wherein the locations are at opposed ends of the casing.
 6. Audioapparatus according to claim 1, wherein the cavity has a mechanicalimpedance (Z_(cavity)) which is lower than the output impedance of thetransducer.
 7. Audio apparatus according to claim 1, wherein the cavityhas a mechanical impedance lower than the impedance of the pinna(Z_(pinna)).
 8. Audio apparatus according to claim 1, wherein thecoupling provides a contact pressure between the pinna and the apparatusso that the apparatus is coupled to the full mechanical impedance of thepinna.
 9. Audio apparatus according to claim 1, wherein the coupling isin the form of a hook, an upper end of which curves over an uppersurface of the pinna.
 10. Audio apparatus according to claim 9, whereina lower end of the hook curves under the lower surface of the pinna. 11.Audio apparatus according to claim 9, wherein the housing is mounted tothe hook so that the transducer casing contacts a lower part of thepinna.
 12. A method of designing audio apparatus comprising mechanicallycoupling a piezoelectric transducer to a user's pinna and driving thetransducer so that the transducer excites vibration in the pinna tocause it to transmit an acoustic signal from the transducer to a user'sinner ear, characterised by embedding the transducer in a casing ofrelatively soft material and by mounting the casing to protectivehousing of relatively hard material such that a cavity is definedbetween the casing and housing.
 13. A method according to claim 12,comprising selecting parameters of one or more of the cavity, casing andhousing to reduce unwanted radiation, to provide protection for thetransducer and/or to ensure good sensitivity and bandwidth.
 14. A methodaccording to claim 13, wherein the coupling between the casing andhousing and/or the cavity is selected to reduce unwanted radiation. 15.A method according to claim 13, wherein the mechanical impedance of thecavity is selected to be lower than the output impedance of thetransducer.
 16. A method according to claim 15, wherein the mechanicalimpedance of the cavity is selected to be lower than the impedance ofthe pinna.
 17. A method according to claim 12, comprising measuring theacoustic performance of the audio apparatus for each user and adjustingthe location of the transducer on the pinna for each individual user tooptimise acoustic performance.
 18. A method according to claim 17,wherein the optimal position is measured by determining the anglebetween a horizontal axis extending through the entrance to the earcanal and a radial line which extends through the entrance and whichcorresponds to the central axis of the transducer.